1. Field of the Invention
The present invention relates generally to electrical circuits and more specifically to devices to protect electrical circuits from damage due to polarity reversal of the power supply.
2. Description of the Prior Art
The earliest devices which provide protection for circuitry from polarity reversal are electromechanical in nature. U.S. Pat. No. 3,267,301 issued to Gignac discloses such a circuit using a relay for polarity reversal protection. Because of the reliability limitation of relays, subsequent designs often included diodes such as disclosed in U.S. Pat. No. 3,313,995 issued to Bach et al.
Subsequent polarity reversal protection circuits used semiconductor devices to replace the electromagnetic relays. These circuits such as taught by Hurd in U.S. Pat. No. 3,571,608, Maigret et al in U.S. Pat. No. 3,829,709 and Moore in U.S. Pat. No. 3,819,951 use transistors to affect the switching of power source to load current.
Because of the high current requirements of polarity reversal protection circuits, the semiconductors used in the prior art are typically bipolar in nature. This results in a constant voltage drop across the polarity reversal protection circuit within the intended current range of the system. For relatively high power systems, this is acceptable. However, for many low power applications, a smaller and arbitrarily lower voltage drop is desirable or required. Even though Moore, for example, suggests the use of metal oxide semiconductors, the prior art does not teach a polarity reversal protection circuit having an arbitrarily low voltage drop.